Fiji's last democratically elected leader was sentenced on Friday to one year in prison after being convicted on corruption charges that his supporters say were politically motivated.

Laisenia Qarase was found guilty on Tuesday of abusing his office and failing to perform his duties. Court documents say the 71-year-old former prime minister failed to disclose a private investment interest in three companies that he was helping manage in an official capacity some 20 years ago.

Qarase served six years as prime minister before being ousted by Fiji's current leader, Commodore Frank Bainimarama, in a 2006 coup. At the time of the offences, Qarase was financial adviser for the Fijian Affairs Board and the now-disbanded Great Council of Chiefs, as well as a company director.

More than 300 Qarase supporters showed up at his sentencing at the high court in the capital, Suva, where security was tight. Many of his supporters maintain the charges were a politically motivated way for Bainimarama's regime to consolidate power and prevent his foes from running for office.

Bainimarama has made clear his disdain for Qarase and the former prime minister's political policy of favouring native Fijians ahead of other ethnic groups.

Bainimarama has promised to hold free elections in 2014, and Fiji is drafting a new constitution ahead of the polls. The new constitution is expected to include provisions to prevent convicted criminals from running for office, meaning the convictions will likely end Qarase's political career.

The sentencing came just days after New Zealand and Australia decided to restore full diplomatic ties with Fiji, following a Sydney meeting among the foreign ministers of the three nations. Bainimarama's promise to hold free elections and his moves to create a new constitution for the island nation had helped ease tensions.

At Qarase's sentencing, Judge Priyantha Fernando said the case should serve as a lesson to others in positions of power.

"Your actions are not only illegal, but against moral values," the judge said. "The punishment that you ought to receive must reflect the breach of public confidence."

The court, however, said it reduced Qarase's sentence by 18 months due to mitigating factors, including Qarase being a first-time offender who suffers from worsening diabetes.